The colour is a function of what is underneath. Woljeongri's sand is pale and the shallows are floored with more of it rather than with rock or weed, so light goes down, bounces off white sand and comes back up — which is what produces that particular turquoise rather than the darker green of most Jeju bays.
The beach is short, maybe a few hundred metres, and the swimming is fine but unremarkable. What Woljeongri does better than anywhere on the island is the thing in between: sitting on a terrace with a coffee, looking at the water and the wind turbines on the headland.
Note that camping and surfing are both prohibited here. If either is your plan, Gimnyeong a few minutes east has a campsite and Hamdeok twenty minutes west has a free one.